The addition of elements acting as grain refiners to various solid solution, single-phase alloys has been used for the purpose of maintaining a fine grain size in the alloy during processing from the original casting to the final wrought product, thus serving to improve processing steps and/or to attain improved properties. In most cases, the grain refiner tends to maintain uniform alloy properties over a range of compositions and of processing conditions. At times, however, as with copper base alloys containing aluminum, zinc, and a grain refining element such as cobalt, the grain refining action is not adequately effective over the full range of operating temperatures up to the melting point of the alloys.
Copper base alloys containing grain refiners generally tend to maintain a fine grain size over a range of annealing temperatures, and display relatively small variations in mechanical properties in these ranges. While this is a desirable feature, it is accompanied by definite restrictions in the normally available ductility of the alloy. In contrast thereto, when a solid solution, single-phase alloy without grain refiners is subjected to higher annealing temperatures, the grain size and the ductility of the alloy increase and the strength decreases.
It is common practice to anneal at the highest temperature consistent with strength requirements to obtain material which requires unusually high ductility in forming operations such as stretch forming. The annealing temperature is further limited for fabricating parts which require a highly polished surface in that above a certain grain size, an "orange peel" condition occurs during fabrication which detracts from the appearance of the polished surface.
It is an undesirable feature of many grain refined copper base alloys that any attempt to coarsen the grain size above the stable level imposed by the grain refining addition results in an uncontrolled mixed grain size consisting of very small and abnormally large grains. Such irregular grain growth is caused by factors such as secondary recrystallization which are a direct result of the effect of the second phase particles on the matrix during cold working and subsequent annealing. Material subjected to irregular grain growth is not suitable for fabrication into parts requiring smooth surfaces for buffing and electroplating and is also characterized by nonuniformity of mechanical properties.